You're looking for the material column.
If you have a database editor like HeidiSQL or Navicat it's pretty simple. If not a simple SQL query like this:
Code:
UPDATE items SET material = 'XX' WHERE id = 'YY';
XX is the material value and YY is the item id
Materials primarily range from 0-24, but there are a few that fall higher than that, like custom helm graphics (Custom Cowl of Mortality) and some kunark helms. There's no complete list that I know of.